Nanocomposites are polymers reinforced with nanometer sized particles, i.e., particles with a dimension on the order of 1 to several hundred nanometers. When nanoparticles are dispersed homogeneously throughout the polymer matrix, dramatic improvements in properties such as strength, flexural and Young's modulus, heat distortion temperature, conductivity, bioactivity, and barrier to gas permeation can be observed at very low filler loadings (<10% by weight). The nature and degree of property improvements depend in part on the geometry of the nanoparticle, its surface chemistry, and its interaction with the polymer matrix. When the nanoparticle filler is well-dispersed within the matrix, few or no nanoparticle aggregates are formed and the total surface area between the filler and the matrix is roughly equivalent to the sum of the surface areas of the individual filler particles. When the nanoparticles are not fully dispersed but are present as aggregates in the polymer matrix, optimum particle properties may not be realized.
Several techniques have been used to produce well-dispersed nanocomposites, such as precipitating small particles from a fluid dispersion using a nonsolvent (also referred to as an “antisolvent” at or near supercritical conditions; adding a solution/dispersion to a nonsolvent dropwise; using nonmiscible solvents, such that an emulsion is formed when the mixture is added to the second solvent phase; and adding a dispersion of nanoparticles in a solvent to a polymer-solvent mixture, where the solvent for the dispersion is a nonsolvent for the polymer.
Winey et al. in U.S. Pat. No. 7,759,413 disclose the preparation of nanocomposites whereby a nanofiller, such as single walled carbon nanotubes, is dispersed in a solvent; a polymer is dissolved in that same solvent; and the resulting mixture is added dropwise or all at once to a second fluid that is a nonsolvent for the polymer. Mixing with the nonsolvent occurs kinetically by interdiffusion of solvent between the two phases, and undesirable clustering of the particles can occur during this interdiffusion process.
However, to produce well-dispersed nanocomposites with a higher degree of control, it is desirable to produce smaller droplets than a standard dropper, and to have independent control of the ability to precipitate the polymer and coagulate the nanoparticle.
There remains a need for a method that produces well-dispersed nanoparticles encapsulated in a polymer matrix, operates at atmospheric temperature and pressure, and allows for independent control of the precipitation of the particle and of the polymer.